This invention relates to a method for cathodic protection which is particularly but not exclusively arranged for use with reinforced concrete and to an anode construction for use with a method of cathodic protection.
Cathodic protection of steel elements at least partly embedded in a surrounding layer is well known. This is primarily used for protection of large structures such as pipe lines or drilling rigs in a corrosive environment. However proposals have been made for cathodic protection of reinforcing elements in concrete structures where the effect of the cathodic protection may be much more localized and may not act to protect the steel reinforcement as a whole.
It is also known that corrosion of steel in concrete can be reduced or halted by generating movement of ions within the concrete structure between an anode and a cathode defined by the conventional metal reinforcing members within the concrete. Techniques are available for cathodic protection in which sufficient current is generated to maintain an ongoing protection and for restoration in which the current is used for a relatively short time but at a sufficient value to cause restorative effects.
Various restorative effects can be obtained including particularly the extraction of chloride ions from the concrete which would otherwise cause corrosion of the metal reinforcement leading to degradation of this structure and spalling of the concrete material covering the reinforcing members. In this method an electrolyte is carried in a porous material between the outside surface of the concrete and the anode.
Examples of this method are shown and described in detail in a brochure by Norcure Chloride Removal Systems Inc. entitled xe2x80x9cIs Salt Induced Corrosion Causing Problems with your Concrete Structuresxe2x80x9d, in a brochure by Vector Construction entitled xe2x80x9cThe Concrete Restoration and Protection Specialistsxe2x80x9d and in a brochure by xe2x80x9cFosroc/NCTASxe2x80x9d entitled xe2x80x9cNorcure Desalinationxe2x80x9d. These brochures describe a technique which is used for various concrete structures including bridge decks and the brochure by Fosroc shows particularly a technique in which a bridge deck is restored using this anodic method.
In the brochure and as generally used in practice, after the concrete surface is exposed by removal of any covering layers, a porous material is laid down onto the upper surface and this receives an electrolyte. The porous material is then covered by a mesh type electrode in the form of wire netting which is then covered by a further layer of the porous material.
A current supply is connected between the mesh anode and the reinforcing steel of the concrete and over an extended period of many weeks this acts to cause the transfer of ions from the concrete material through the electrolyte to provide a restorative effect.
The increased usage of salt as a de-icing agent in freezing conditions has severely exacerbated the problem of chloride degradation of concrete structures in climates where freezing conditions can be expected. Also the presence of salt in a marine environment can generate similar degradation.
Restoration of concrete using a temporary current is an entirely different process from impressed current cathodic protection. In the latter process, a small current typically of the order of 1-10 mAmps/sq meter is caused to flow continuously through the life of the concrete for the purpose of inhibiting corrosion.
The current used in the restoration process is strictly temporary for a period of the order of 20 to 90 days and has a value which is of the order of 50 to 200 TIMES that of the continuous current. Thus the current in the restoration process may lie in the range 0.4 to 3.0 Amps/sq meter. In addition, the process of restoration must include a liquid electrolyte whereas the continuous process is typically dry. Therefore the types of anode and materials to be used are of an entirely different character.
In PCT Published Application WO94/29496 of Aston Material Services Limited is provided a method for cathodically protecting reinforcing members in concrete using a sacrificial anode such as zinc or zinc alloy. In this published application and in the commercially available product arising from the application, there is provided a puck-shaped anode body which has a coupling wire attached thereto. In the commercially available product there are in fact two such wires arranged diametrically opposed on the puck and extending outwardly therefrom as a flexible connection wire for attachment to an exposed steel reinforcement member.
The puck is surrounded by an encapsulating material such as mortar which holds an electrolyte that will sustain the activity of the anode. The mortar is compatible with the concrete so that electrolytic action can occur through the mortar into and through the concrete between the anode and the steel reinforcing member.
The main feature of the published application relates to the incorporation into the mortar of a component which will maintain the pH of the electrolyte in the area surrounding the anode at a high level of the order of 12 to 14.
In use of the device, a series of the anodes is provided with the anodes connected at spaced locations to the reinforcing members. The attachment by the coupling wire is a simple wrapping of the wire around the reinforcing bar. The anodes are placed in locations adjacent to the reinforcing bars and re-covered with concrete to the required amount.
Generally this protection system is used for concrete structures which have been in place for some years sufficient for corrosion to start. In general, areas of damage where restoration is required are excavated to expose the reinforcing bars whereupon the protection devices in the form of the mortar-covered puck are inserted into the concrete as described above and the concrete refilled.
These devices are beginning to achieve some commercial success and are presently being used in restoration processes. However improvements in operation and ergonomics are required to improve success of this product in the field.
In International Publication W098/16670 of Bennett and Clear is disclosed another cathodic protection system intended to be used as a surface arrangement. This arrangement relates to a thinly sprayed zinc or zinc alloy which is applied onto the surface of the concrete. This zinc or zinc coating is then used as an anode to supply current for the cathodic protection process. As the anode is exposed at the surface, this may be used either as a sacrificial system in which there is no applied current and the anode is gradually corroded as the electrolytic process proceeds or as an impressed current cathodic protection system.
The improvement of the above Bennett application relates to the application of a humectant in free-flowing form which is positioned at or near the interface between the zinc anode coating and the concrete surface. It has been found and is disclosed in this application that the provision of the humectant in free-flowing form acts to absorb moisture from the area above the surface. The humectant is defined in the application as being either deliquescent or hygroscopic where a deliquescent material is one which becomes moist or liquefied after exposure to humid air and a hygroscopic material is defined as one which is capable of absorbing moisture from the atmosphere. The humectant is delivered to or near the interface of the anode by application as a solution which is aqueous, colloidal or in an organic solvent such as alcohol. When the humectant in solution is applied to the surface of the anode, it is transported to or near the interface by capillary action. The application states that the humectant is applied to the exposed surface of the anode coating and therefore the anode coating must be sufficiently thin or otherwise arranged to be porous to allow the humectant to reach the interface.
U.S. Pat. No. 4,265,725 (Tatum) assigned to CE Equipment and issued May 5, 1981 discloses an arrangement for making a rigid connection between an anode and an electrical connector therefor.
U.S. Pat. No. 5,609,748 (Kotowski) assigned to Heraeus Elektroden and issued Mar. 11, 1997 discloses an anode arrangement to be buried within a concrete structure to provide cathodic protection.
U.S. Pat. No. 5,431,795 (Moreland) assigned to Thoro Systems Products and issued Jul. 11, 1995 discloses a cathodic protection system which uses an alkaline buffer to prevent acid build up for use with an electrically conductive coating on a concrete structure.
U.S. Pat. No. 6,193,857 (Davison) assigned to Foseco discloses an anode body in the form of a puck coated with a mortar in which the puck is attached by ductile wires to the rebar within an excavation in the concrete.
It is one object of the present invention, therefore to provide an improved method for cathodic protection.
According to a first aspect of the invention there is provided a method for cathodic protection comprising:
providing a covering material and a steel member at least partly embedded in the covering material;
providing a sacrificial anode body in the form of a solid body separate from the covering material;
electrically connecting the anode body to the steel member so that an electrical potential therebetween causes an electrical current to flow therebetween through the electrical connection and causes ions to flow through an interface of the anode body and through the covering material tending to inhibit corrosion of the steel member;
and providing a material which is bound into the anode body so as to be carried thereby or which is bound into a material surrounding the anode body so as to be carried thereby which acts in the presence of moisture to communicate ions at the interface of the anode body to keep the interface electrochemically active during the life of the anode body to maintain said cathodic protection, where the anode body is covered by the covering material and/or the material surrounding the anode body;
and providing in the material the characteristic of a humectant and causing the presence of the humectant material to absorb additional moisture sufficient to maintain conductivity at the interface to a level greater than would occur in the absence of the humectant material.
Preferably the humectant material is carried in a manner which allows a surface of the material of the anode body to communicate ions and which presents the humectant material at the surface of the anode body.
The humectant material is one example only of enhancement materials which will effect an enhancement of the ion communication over an extended period. These can include but are not limited to the alkali described in more detail hereinafter.
In one alternative, the anode body itself carries the humectant, or other enhancement material, which is incorporated into the anode body. The incorporation can be effected as an admixture or commingled with the zinc or other sacrificial material as it is cast in molten form. Otherwise the material can be incorporated by techniques such as dividing the material of the anode and the humectant, or other enhancement material, and admixing the divided materials into a solid integral body by sintering or pressure or other suitable method. Yet further, the enhancing material can be encapsulated with the anode material by folding or rolling the material into a foil of the anode material. The mixture is effected so that the above condition applies at the finished surface of the anode body. Normally the enhancement material will be a salt or an organic polymer and not an element and not a metal. It is not intended therefore that the term xe2x80x9cenhancement materialxe2x80x9d used herein should include metals or other elements which are used in an alloy with the sacrificial material of the anode. Thus the enhancement material is admixed or commingled with the sacrificial anode material either mechanically or in a molten state of the anode material and not in an alloying process.
In another alternative, the anode body comprises a core body of a sacrificial material and a layer permanently attached to at least one outer surface of the core body thus defining an anode member separate from the covering material for embedding in the covering material, the layer being arranged to allow communication of ions through the covering material between the core body of the anode member and the steel member and wherein the humectant material is bound into the layer as a mixture therewith. Preferably the layer is a solid such as a cementitious material cast on the outside of the sacrificial anode body.
Preferably the anode body is buried in the covering material so as to be wholly embedded therein. This can be achieved by excavating within the existing structure and filling the excavation after the anode is inserted. Alternatively the anode can be covered or buried by applying onto an existing layer of concrete a covering layer. Thus the anode may be only partly buried in the original concrete or may be wholly outside the original concrete and thus may be covered by the new concrete applied. In this way, in some cases, no excavation of the original material may be necessary. The additional concrete can be applied by attaching a suitable form, for example a jacket similar to that shown in U.S. Pat. No. 5,714,045 (Lasa et al) issued Feb. 3, 1998. The form shown in this patent is particularly designed for columns but other arrangements could be designed for other structures. The anode shown in this patent is replaced by the anodes disclosed hereinafter.
Preferably the humectant material is a solid.
Preferably the method includes the steps of forming at least one hole in the covering layer, preferably by drilling since only a relatively small hole is required, so as to expose the steel member therein, inserting the anode body into the hole, attaching the anode body to the steel member and at least partially filling the hole.
In a yet further alternative, the method includes the steps of forming at least one hole in an existing layer of covering material so as to expose the member therein, inserting the anode body into the hole or one of the holes, electrically connecting the anode body to the steel member in the hole or another hole and at least partially filling the hole with a filler material separate from the anode body and wherein the humectant material is contained in the filler material as a mixture therewith.
Preferably the method includes providing a material which is bound into the anode body so as to be carried thereby or which is bound into a material surrounding the anode body so as to be carried thereby which provides at least at the surface of the anode body a pH greater than 12 and preferably greater than 14.
Preferably the anode body is electrically connected to the steel member by a solid pin rigidly attached to the steel member.
In one alternative, the pin has one end driven into the steel member by an impact tool.
In another alternative, the pin has one end electrically welded to the steel member.
In a yet further alternative, the anode member is electrically connected to the reinforcing member by a connecting member having a flowable metal portion attached to the anode member by forces causing the flowable portion to flow.
In one arrangement which is particularly of advantage where the thickness of the concrete layer is relatively thin, said at least one hole includes a first and a second hole, wherein the anode member is inserted into the first hole, wherein the second hole is in communication with a steel member and wherein an electrical connection from the anode member is rigidly attached to the steel member in the second hole.
The pin may include at least a portion extending into the anode body either wholly or partly through the body in the longitudinal direction.
According to a second aspect of the invention there is provided a method for cathodic protection of a concrete structure comprising:
providing an existing concrete structure having a steel member and a layer of concrete covering the steel member so as to define a surface of the concrete layer spaced from the steel member;
providing a sacrificial anode member in the form of a solid body separate from the concrete layer;
forming at least one hole in the existing layer of concrete so as to expose a steel member therein;
inserting the anode member into the at least one hole;
and electrically connecting the anode member to the exposed steel member;
the anode body being electrically connected to the steel member so that an electrical potential therebetween causes an electrical current to flow therebetween through the electrical connection and causes ions to flow through the concrete layer tending to inhibit corrosion of the steel member
wherein the at least one hole is formed by drilling and the anode member is shaped for insertion into the at least one drilled hole.
In one alternative, the anode member may be shaped and dimensioned so that when inserted into the hole it has a cylindrical outer surface which is a tight fit on the wall of the drilled hole. This can be achieved by providing a diameter which matches or slightly exceeds that of the hole so that the anode member is driven into the hole. Alternatively, the member may be dimensioned as a free sliding fit within the drilled hole and is expanded into a tight fit within the drilled hole. This can be done by impact forces or pressure acting to drive the anode member into the hole or by otherwise expanding the anode member for example by an insert driven into the anode member. In a further alternative, the anode member may be driven into place by a tool shaped to match the top or exposed face but which includes a pattern in relief which forms an embossed pattern in the face to confirm to the installer that sufficient force has been applied to drive the member to the required position and to bottom it against the rebar, and if necessary to expand into a tight fit.
In this arrangement, the anode member may include an electrical connector at the bottom end and the electrical connector is driven into connection with the steel member by said driving forces.
According to a third aspect of the invention there is provided an anode member for use in cathodic protection of a steel member in a covering material, the anode member comprising:
a solid anode body separate from the covering material formed by a sacrificial anode material;
an electrical connecting member for electrical connection to the steel member;
and an enhancement material for co-operating with the sacrificial anode material in enhancing the communication of ions between the covering material and the anode material,
wherein the enhancement material is bound into the sacrificial anode material of the solid anode body so as to be carried thereby.
In one preferred arrangement, the enhancement material and the sacrificial anode material, such as zinc, in divided form are pressed together to form a porous body. This arrangement has the advantage that corrosion products from corrosion of the zinc anode body are received into pores of the porous zinc body.
According to a fourth aspect of the invention there is provided an anode member for use in cathodic protection of a steel member in a covering material, the anode member comprising:
a solid anode body separate from the covering material formed by a sacrificial anode material;
an electrical connecting member for electrical connection to the steel member;
wherein the electrical connection includes a solid member rigidly connected with the anode body so as to be exposed at a bottom surface of the anode body, the solid member being arranged for rigid attachment to the steel member.
According to a fifth aspect of the invention there is provided a method for cathodic protection comprising:
providing a covering material and a steel member at least partly embedded in the covering material;
providing a sacrificial anode member in the form of a solid body separate from the covering material;
embedding the anode body in the covering material;
and electrically connecting the anode member to the steel member;
the anode body being electrically connected to the steel member so that an electrical potential therebetween causes an electrical current to flow therebetween through the electrical connection and causes ions to flow through the covering material tending to inhibit corrosion of the steel member
wherein the anode body is formed of a sacrificial anode material which is arranged to define a porous body having pores therein;
and preventing expansion of the anode body during corrosion by arranging the pores in size and number such that corrosion products from corrosion of the anode body are received into the pores.